1. Field of the Invention
The present invention relates to a coating and developing system for processing substrates, such as semiconductor wafers or LCD substrates (glass substrates for liquid crystal displays) by a cleaning process, a coating process for coating the substrate with a liquid resist and a developing process for processing the substrate processed by an exposure process, and to a coating and developing method.
2. Description of the Related Art
A system built by connecting an exposure system to a coating and developing system carries out a series of processes for forming a desired resist pattern on a substrate, such as a substrate on which a semiconductor device is to be fabricated or an LCD substrate, by applying a liquid resist to a surface of the substrate in a resist film, exposing the resist film through a photomask and developing the exposed resist film.
The coating and developing system includes a cassette station provided with a cassette table for supporting a wafer cassette and a transfer device which takes out a semiconductor wafer (hereinafter, referred to simply as “wafer”) from and puts a wafer into the cassette, a processing station for processing the wafer by resist film forming processes and a developing process, and an interface station connected to an exposure system. The cassette station, the processing station and the interface station are arranged in a row in that order.
After the resist pattern has been formed, the wafer is inspected by a predetermined inspection procedure for measuring the width of lines forming the resist pattern, examining the alignment of the resist pattern with a pattern underlying the resist pattern and finding defects caused by development. Only acceptable wafers are transferred to the next process. In most cases, the inspection is carried out by a stand-alone inspection system separate from the coating and developing system. However, inspection using an in-line inspection system included in the coating and developing system is more convenient than inspection using the stand-alone inspection system.
A substrate processing system proposed in JP-A 2005-175052 (Paragraph 0042 and FIG. 4) includes a cassette station, a processing station, and an inspection station including a plurality of inspection devices and carrying devices and interposed between the cassette station and the processing station. The processing system mentioned in JP-A 2005-175052 carries a substrate from the cassette station through the inspection station to the processing station, returns the completely processed substrate temporarily to a cassette placed in the cassette station, and then carries the processed substrate from the cassette to the inspection station to inspect the processed substrate.
FIG. 9 is a schematic plan view of a coating and developing system concretely realizing this previously proposed processing system. Shown in FIG. 9 are a cassette station 11, an inspection station 12, a processing station 13, and an interface station 14 connected to an exposure system. Shown also in FIG. 9 are wafer cassettes C, a transfer device 15 placed in the cassette station 11, a carrying device 16 placed in the inspection station 12, transfer modules TRSa, TRSb, TRSc and TRSd, and inspection modules E1, E2 and E3. Although the transfer modules TRSa, TRSb, TRSc and TRSd, and the inspection modules E1, E2 and E3 are shown in a planar arrangement for convenience, actually, the transfer modules TRSa, TRSb, TRSc and TRSd are stacked in four layers, and the inspection modules E1, E2 and E3 are stacked in three layers.
In the coating and developing system shown in FIG. 9, a wafer is carried from the cassette C through the transfer device 15, the transfer module TRSa and the carrying device 16 in that order to the processing station 13. Processing modules installed in the processing station process the wafer by processes for coating a surface of the wafer with a resist film. The wafer having the surface coated with the resist film is delivered through the interface station 14 to the exposure system. After the wafer has been processed by the exposure system, the wafer is returned to the processing station. The processing modules of the processing station 13 process the wafer by developing processes. Then the wafer is returned through the carrying device 16, the transfer module TRSb and the transfer device 15 in that order to the cassette C.
Wafers contained in the cassette C are processed in predetermined order. For example, ordinal numerals first to thirteenth are assigned respectively to thirteen wafers contained in the cassette C. The wafers are carried in numerical order to the processing station 13. The wafers are carried successively to the modules in predetermined order. The carrying device, namely, the main carrying device, of the processing station 13 performs a cyclic carrying operation to carry the wafers cyclically in a cyclic carrying mode through a series of the modules in predetermined order. Thus the wafers are carried successively in the predetermined order through the series of the modules. The main carrying device has two arms for replacing the wafer held in the module with other one. Suppose that a carrying route to be followed by the main carrying device is a circulation route. Then, the carrying device travels once through the circulation route in a predetermined cycle time. The carrying device does not travel back and does not carry wafers in a skip carrying mode, in which the wafer specified by a larger ordinal numeral, namely, the wafer taken out later from the cassette C, is carried ahead of the wafer specified by a smaller ordinal numeral, namely, the wafer taken out earlier from the cassette C. The carrying device operates in such a carrying mode because an impractical, complicated carrying program is necessary for controlling the carrying device unless the carrying device operates in such a carrying mode.
All the wafers returned to the cassette C or chosen ones of the wafers returned to the cassette C are delivered to the transfer module TRSc. The carrying device 16 carries the wafers from the transfer module TRSc to the inspection module. Some of the wafers are inspected only by the inspection module E1, E2 or E3 and some other wafers are inspected successively by the inspection modules E2 and E3. The carrying device 16 carries the wafers in the inspection station 12 in a cyclic carrying mode similar to that in which the main carrying device carries the wafers in the processing station 13. The carrying device 16 operates in the cyclic carrying mode in synchronism with the cyclic carrying operation of the main carrying device.
The wafer delivered to the transfer module TRSa needs to be held in the transfer module TRSa until the main carrying device comes to a transfer module, not shown, in the processing station 13 to receive the wafer, because the main carrying device advances without receiving any wafer from the transfer module and the carrying of the wafer is delayed by one carrying cycle. Therefore, the carrying device 16 working in the inspection station 12 needs to give the highest priority to carrying wafers between the transfer modules TRSa and TRSb, and the processing station 13. Under such a condition, the carrying device 16 performs the cyclic carrying operation to carry a wafer through the transfer module TRSc, the inspection modules E1, E2 and E3 and the transfer module TRSd in that order after completing carrying a wafer to the transfer module TRSa or TRSb. For example, if the inspection operations for inspecting wafers by the inspection modules E1 and E2 are accomplished simultaneously, one of the wafers is held in the inspection module E1 (or E2) for a time corresponding to one carrying cycle.
Consequently, the throughput of the inspection station 12 decreases. The inventors of the present invention made studies to operate the carrying device 16 asynchronously with the main carrying device. When the carrying device 16 operates asynchronously with the main carrying device, the carrying device 16 performs preferentially a carrying operation for delivering a wafer to the processing station or a carrying operation for receiving a wafer from the processing station when a ready signal is provided by the transfer module TRSa or when a ready signal is provided after a processed wafer has been delivered to the transfer module of the processing station 13. The carrying device 16 can use whole of the rest of the time for transferring a wafer to and receiving the wafer from one of the inspection modules E1, E2 and E3.
However the following problems arise when the carrying device 16 operates in such a mode. Suppose that the coating and developing system is required to process 150 wafers every hour, the transfer device 15 takes 8 s to complete the transfer operation once and the carrying device 16 takes 5 s to complete the carrying operation once. Then, the cycle time of the cyclic carrying operation should not be longer than 24 s (=3600 s/150) to process 1500 wafers every hour. Consequently, the transfer device 15 can accomplish the carrying operation only three times (24/8=3) during one carrying cycle and the carrying device 16 can accomplish the carrying operation only four times (20/5=4) during one carrying cycle.
As mentioned above, wafers need to be carried preferentially to the transfer modules TRSa and TRSb. Therefore, the transfer arm 15 can accomplish the transfer operation with the transfer modules TRSc and TRSd once during one carrying cycle. The carrying dev ice 16 can accomplish the carrying operation with the inspection module E1 (E2, E3) twice during one carrying cycle. The inspection modules E1, E2 and E3 carry out different inspections, respectively, and the different inspections take different inspection times, respectively. Suppose that the inspection Modules E1, E2 and E3 need inspection times of 30 s, 100 s and 140 s, respectively, and the 11th wafer, the 10th wafer and the 9th wafer are held in the inspection modules E1, E2 and E3, respectively. The 11th wafer cannot be taken out from the inspection module E1 unless the inspection of the 9th wafer is completed before the completion of the inspection of the 11th wafer.
For example, it is possible that the first wafer of the succeeding lot is delivered to the inspection module E3 first, and then the last wafer of the preceding lot is delivered to the inspection module E3 if carrying wafers in the skip carrying mode, in which a wafer specified by a larger ordinal numeral is carried ahead of a wafer specified by a smaller ordinal numeral, is permitted in the inspection station. Consequently, a recipe for an inspection to be carried out by the inspection module E3 needs to be changed and wafers cannot be smoothly carried.
If a wafer cannot be taken out from the inspection module after the completion of inspection, an inspection process takes a long time, which will be explained in connection with the description of a comparative example.